At present, a wide spread attention is attracted to an organic electroluminescent device for next-generation flat panel displays, and such organic electroluminescent device is now being utilized for a sub-display of cell phones and others. An organic electroluminescent device (which is hereinafter referred to “organic EL device” when appropriate) has a multilayer structure comprising a hole transport layer, an emitting layer and an electron transport layer, which are sandwiched between a cathode and an anode. At present, a multilayer having a similar arrangement but additionally having a hole injection layer between the cathode and the hole transport layer, and an electron injection layer between the anode and the electron transport layer is widely adopted to reduce power consumption of the organic EL device and prolong life time thereof.
As a hole transport material, triarylamine derivatives having a diamine structure such as, for example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD) and N,N,N′,N′-tetra[(1,1′-biphenyl)-4-yl]benzidine (TBDB) were reported (see, for example, Advanced Materials, Germany, 1998, Vol. 10, No. 14, p 1108-1112, FIG. 1 and Table 1; and Journal of Luminescence, Netherland, 1997, 72-74, p 985-991, FIG. 1).
In recent years, development of new organic emitting materials is eagerly desired for more enhancing the efficiency and life time of an organic EL device.
An organic electroluminescent material having a benzofluorene structure has been proposed in, for example, WO 2004/61048, claims). However, this patent document is silent on a compound having a benzofluorene structure to which an amino group has been directly bonded. This patent document describes that a polymeric material having a benzofluorene group is useful as an emitting material, but teaches nothing about the use of such polymeric material as a hole transport material and a hole injection material.
Various structures are known for amine compounds having a benzofluorene structure (see, for example, JP 2008-50337 A, claims; and JP 2008-201769, claims). However, an amine derivative represented by the general formula (1), shown below, is not known and processing properties of the amine derivative of formula (1) which are important for producing an organic EL device by a vacuum deposition method are not suggested in these patent documents.